Abstract: A novel relative humidity sensor that is based on a linear piezoelectric micromachined ultrasonic transducer (pMUT) array was proposed and microfabricated for high sensitivity, fast response, and good stability. The humidity-sensitive graphene oxide (GO) film was deposited on the pMUT array with a facile drop-casting method and characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and Fourier transform infrared spectrum (FTIR). With the humidity level ranging from 10% to 90% RH, the sensor exhibited a high sensitivity of 719 Hz/% RH and an extremely high relative sensitivity of 271.1 ppm/% RH. The humidity-sensing results also showed good short-term repeatability and long-term stability, fast response and recovery, and low hysteresis. Moreover, the temperature coefficient of frequency (TCF) of the present humidity sensor was investigated and it could be easily compensated owing to the pMUT array structure design. This work confirmed that the GO functionalized pMUT is an excellent candidate in humidity detection and it may enable many potential applications, such as ultrasensitive mass detection and simultaneous detection of multiple parameters. Keywords: piezoelectric micromachined ultrasonic transducer; humidity sensor; graphene oxide; array structure; high sensitivity

1. Introduction Miniaturized humidity sensors enabled through micro-/nano-fabrication technologies have gained extensive attention due to distinctive advantages of small size, low cost, fast response, high sensitivity, and high stability. These micromachined humidity sensors have great potential for portable electronic systems and they play crucial roles in a wide range of applications, such as environmental monitoring, industrial process control, agricultural production, and medical treatments [1,2]. In general, there are a variety of humidity sensing principles, mainly including optical methods [3,4], electrical methods (resistance [5], capacitance [6], and impedance [7]), and resonant Sensors 2018, 18, 4352; doi:10.3390/s18124352

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mechanical methods (surface acoustic wave (SAW) resonator [8], film bulk acoustic resonator (FBAR) [9], quartz crystal microbalance (QCM) [10], micro-/nano-cantilever [11], and capacitive micromachined ultrasonic transducer (cMUT) [12]). When compared with other humidity sensing technologies, cMUT as an emerging ultrasound generator and detector proposed by Kuri-Yakub group has been proven to be an ultrasensitive and powerful approach for mass load detection, combining low effective mass and quite large active surface [13]. Benefitting from the extremely high mass sensitivity at the zeptogram (zg) scale, the cMUT-based chemical sensor with a selectively sensitive layer exbibits an excellent sensing performance towards water vapor [12]. However, such good performance relies heavily on very narrow cavity (~40 nm) and high bias voltage (~50 V), which largely increases the process difficulty and fabrication cost and further limits its application fields. Similar to the array structure and resonant operation mode of cMUT, piezoelectric micromachined ultrasonic transducer (pMUT) operating at only several volts without a specially designed cavity can also be easily functionalized for chemical and physical sensing by coating the selective sensing material on the device surface. Due to its inherent mass-sensing property, multi-sensor array configuration, and manufacturing advantages when compared with the existing QCM, SAW, and FBAR piezoelectric sensors, the pMUT array is highly promising and much advantageous to provide high humidity sensitivity. However, the humidity sensor that is based on the pMUT has not been investigated yet to our best knowledge. It is well known that humidity-sensing properties, such as sensitivity, response, and stability are largely affected by the selected sensitive thin film. Until now, various kinds of humidity-sensing materials have been employed, including metal oxides [14,15], ceramics [16,17], carbonic materials [18,19], polymers [20,21], and their composites [22–24], etc. Among these materials, graphene oxide (GO), as a typical chemical derivative of graphene, has attracted great interest due to its layered honeycomb structure, containing abundant reactive oxygen functional groups (carboxyl, hydroxyl, and epoxy groups) in each layer [25]. The existence of these hydrophilic oxygen groups makes GO an excellent candidate for humidity detection. Previous studies have demonstrated high sensitivity, fast response and little hysteresis of GO-based resonators [19,26]. Besides, owing to its electrical insulation characteristic, GO can be directly deposited on the electrodes. In this work, a linear pMUT array based humidity sensor functionalized with GO sheets were proposed and fabricated with the microelectromechanical system (MEMS) technology. The GO thin film was directly deposited on the microfabricated pMUT array with a simple and facile drop-casting method and was characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and Fourier transform infrared spectrum (FTIR). The humidity sensing characteristics of developed sensors, such as response/recovery, hysteresis, stability, and temperature effect, were investigated and discussed. Impedance and phase curves at different RH levels were also used to characterize the GO coated pMUT humidity sensor. Last, the sensing mechanism of the present sensor was analysed. This research demonstrates the potential of the pMUT based humidity sensor for highly sensitive mass detection in a wide range of applications. 2. Experimental 2.1. Design and Fabrication of pMUT Humidity Sensor The pMUT based humidity sensor contains a pMUT linear array that was fabricated on a released silicon-on-insulator (SOI) wafer and a GO thin film deposited on the pMUT array. The schematic structures of the pMUT array and pMUT based humidity sensor are illustrated in Figure 1a,b. The pMUT linear array consists of 15 rectangular pMUT elements with a good frequency consistency, some of which are coated with GO film for humidity sensing, while others are uncoated for reference. The pMUT is designed with the morphotropic phase boundary composite lead zirconate titanate (Zr/Ti = 52/48, MPB-PZT) piezoelectric material. The dimensions of piezoelectric membrane and the underlying cavity are 120 µm (Width) × 500 µm (Length) × 1.9 µm (Thickness) and 160 µm (Width)

where t,t,L,L,W, W,E,E, and ν are the thickness, length, width, Young’s modulus, Poisson’s ratio, and where ρ, ρ, and ν are the thickness, length, width, Young’s modulus, Poisson’s ratio, and density density of the rectangular membrane. It is shown that the operating frequency of the pMUT is of the rectangular membrane. It is shown that the operating frequency of the pMUT is proportional proportional tomodulus the Young’s inversely proportional to the density.coating Therefore, coating a to the Young’s andmodulus inverselyand proportional to the density. Therefore, a selectively selectively sensitive thin film on the pMUT surface could shift the operating frequency upward or sensitive thin film on the pMUT surface could shift the operating frequency upward or downward, downward, which is mainly dependent on the material properties and thickness of the deposited layer. However, the physisorption or chemisorption of the analyte molecules on the sensitive thin

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which is mainly dependent on the material properties and thickness of the deposited layer. However, Sensors 2018, 18, x FOR PEER REVIEW 4 of 13 the physisorption or chemisorption of the analyte molecules on the sensitive thin film would result in a frequency to the mass or density change, well-known as the mass-loading film woulddecrease result in adue frequency decrease due to the mass or density change, well-known as theeffect. The frequency shift ∆f ofThe the frequency developedshift pMUT humidity sensorpMUT can be humidity estimatedsensor by thecan following mass-loading effect. Δf of the developed be estimated equation [12]: by the following equation [12]: 1 ∆m ∆ f = − 1 f 0 ×Δm (2) Δf = − 2 f 0 × m (2) 2

m

where m and ∆m are the mass of the effective vibration membrane and mass change after absorption of where m and Δm are the mass of the effective vibration membrane and mass change after absorption water molecules, respectively. According to the mechanical resonant frequency Expression (1) and the of water molecules, respectively. According to the mechanical resonant frequency Expression (1) dimensions, the mass sensitivity per unit area of the designed pMUT is estimated at 16 ag/Hz/µm2 . and the dimensions, the mass sensitivity per unit area of the designed pMUT is estimated at 16 ag/Hz/μm2.

2.3. Characterization and Measurement

2.3. Characterization and Measurement The surface morphologies of the pMUT humidity sensor before and after coating GO film were characterized by SEM operated as shown in sensor Figurebefore 2a,b. and To investigate The surface morphologiesat of15 thekV, pMUT humidity after coatingthe GO thickness film were and uniformity of the by GOSEM film, the same volume and concentration GO dispersions was dropped characterized operated at 15 kV, as shown in Figure 2a,b.ofTo investigate the thickness and on ◦ C concentration uniformity theSiGO film, and the same volume GO dispersions on GO the surface of aofflat wafer heated at 60and for 24 h. TheofAFM analysis ofwas thedropped deposited theshown surfacein ofFigure a flat Si 2c,d. wafer It and at 60 seen °C forthat 24 h.the Theresultant AFM analysis of thefilm deposited film and film is is heated obviously GO thin has a GO layered is shown in Figure 2c,d. is obviously seen the resultant GOduring thin film layered and wrinkled structure, which is It associated with thethat exfoliation process thehas GOa film preparation. wrinkled structure, which is associated with the exfoliation process during the GO film preparation. The average thickness of the GO film is about 406 nm, with the surface roughness Rq and Ra of 32.4 nm The average thickness of the GO film is about 406 nm, with the surface roughness Rq and Ra of 32.4 and 27.2 nm, respectively. nm and 27.2 nm, respectively.

Figure 3 shows the Fourier Transformed Infrared (FTIR) spectrum of the GO film in the range Sensors 2018, x FOR PEER by REVIEW 5 of 13 Two 4000–1000 cm−118,obtained a high-resolution FTIR microscope at the transmission mode. Figure 3 shows the Fourier Infrared (FTIR) spectrum of the GO film in the range −1 andTransformed −1 due peaks appeared at 2357 cm 2333 cm to the formation of unique chemical species in the shows theby Fourier Transformed FTIR Infrared (FTIR) spectrum of−1 the GO filmmode. in the Two rangepeaks 4000–1000Figure cm−13obtained a high-resolution microscope at the transmission presence of CO2 −1[28]. A characteristic absorption peak at 1628 cm suggests the presence of −1 and 2333 −1 due to the formation 4000–1000 cmcmobtained by acm high-resolution FTIR microscope at the transmission appeared at 2357 of unique chemical speciesmode. in theTwo presence −1 carboxyl groups (C=O stretching vibrations). Thetostrong and broad valley chemical at 3579 cm is correlated −1 and 2333 cm−1 due − 1 peaks appeared at 2357 cm the formation of unique species in the of CO2 [28]. A characteristic absorption peak at 1628 cm suggests the presence of carboxyl groups to thepresence –OH stretching vibrations. of CO2 [28]. A characteristic absorption peak at 1628 cm−1 suggests the presence of (C=O stretching vibrations). The strong and broad valley at 3579 cm−1 is correlated to the –OH carboxyl groups (C=O stretching vibrations). The strong and broad valley at 3579 cm−1 is correlated stretching vibrations. to the –OH stretching vibrations.

The experimental setup forfor humidity-sensing propertymeasurement measurement is illustrated in Figure 4. The experimental setup humidity-sensing property is illustrated in Figure 4. The experimental setup for humidity-sensing property measurement is illustrated in Figure 4. The pMUT humidity sensors were placed sealedplastic plastic chambers. 2O5 desiccant The pMUT humidity sensors were placedininone one of of two two sealed chambers. TheThe P2OP 5 desiccant The pMUT humidity sensors were placed in one of two sealed plastic chambers. The P O desiccant and 2 5 and humidifier filled with deionized usedtogether togethertoto achieve humidity levels and humidifier filled with deionized(DI) (DI)water water were were used achieve humidity levels fromfrom humidifier filled with deionized (DI)RH water were used together tohumidity achieve humidity levels from 10%was RH 10% RH to 90% RH with a 10% interval. The relative in the test chambers 10% RH to 90% RH with a 10% RH interval. The relative humidity in the test chambers was to 90% RHchanged with a 10% RH interval. The relative humidity in the test chambers was changed manually changed by adjusting the flow rate of of the evaporative moistures and it was recorded in in manually by adjusting the flow rate the evaporative moistures and itmanually was recorded by adjusting the flow rate of the evaporative moistures and it was recorded in real time by a precise real time a precise relative humiditymeter. meter. A A precision precision impedance analyser (Agilent 4294A, real time by abyprecise relative humidity impedance analyser (Agilent 4294A, relative humidity meter. A Singapore) precision impedance (Agilent 4294A,frequency Agilent Technologies Inc., Agilent Technologies Inc., Singapore) wasused usedanalyser to measure measure the of the Agilent Technologies Inc., was to theresonant resonant frequency of pMUT the pMUT humidity sensors the data the wereresonant transmitted to a personal (PC) for saving andand off-line Singapore) was used to measure frequency of thecomputer pMUT humidity the data humidity sensors andand the data were transmitted to a personal computer (PC) forsensors saving and off-line processing. The temperature in the chamber wassaving kept constant at 24 processing. ± 0.5 °C to eliminate the were transmitted to a personal computer (PC) for and off-line The temperature processing. The temperature in the chamber was kept constant at 24 ± 0.5 °C to eliminate the ◦ C to interference from kept temperature changes. Xiaomi Mijia bluetooth temperature sensor in the chamber was constant at 24 A ±A Xiaomi 0.5 eliminate thehumidity interference from temperature interference from temperature changes. Mijia bluetooth humidity temperature sensor (Xiaomi Inc., Beijing, China) was also applied to track both the humidity and temperature values changes.Inc., A Xiaomi Mijia bluetooth humidity temperature sensor (Xiaomi and Inc., temperature Beijing, China) was (Xiaomi Beijing, China) was also applied to track both the humidity values per second. During the experiments, the sensors were placed in one chamber first to obtain the also applied to track both the humidity and temperature values per second. During the experiments, per second. the experiments, the sensors weretransferred placed ininto onethechamber first to obtain the responseDuring at one humidity level, and then were rapidly other chamber to obtain the sensors were placed in one chamber first to obtain the response at one humidity level, and then response at one humidity and level. then In were into the other chamber to obtain the response at anotherlevel, humidity thisrapidly way, thetransferred humidity-sensing response and hysteresis were rapidly transferred into the other chamber to obtain the response at another humidity level. In characteristics could be exploredlevel. by changing relative humidity in two chambers from 10% RH this the response at another humidity In this the way, the humidity-sensing response and hysteresis way, the humidity-sensing response and hysteresis characteristics could be explored by changing the to 90% RHcould and then to 10%by RH. characteristics be back explored changing the relative humidity in two chambers from 10% RH relative humidity in back two chambers from 10% RH to 90% RH and then back to 10% RH. to 90% RH and then to 10% RH.

3. Results and Discussion 3. Results and Discussion 3.1. Performance of pMUT without GO Film 3.1. Performance of pMUT without GO Film The finite element method (FEM) simulation that was based on COMSOL Multiphysics v5.2a The finite element method (FEM) simulation that was based on COMSOL Multiphysics v5.2a was first employed to study the resonant characteristics of the pMUT. Figure 5a shows the predicted was first employed to study the resonant characteristics of the pMUT. Figure 5a shows the predicted response spectrum with a fundamental frequency of 2.58 MHz and a displacement sensitivity of response spectrum with a fundamental frequency of 2.58 MHz and a displacement sensitivity of 80 80 nm/V nm/Vpppp . Subsequently, both the impedance and phase curves of the pMUT humidity sensors were . Subsequently, both the impedance and phase curves of the pMUT humidity sensors were measured before and plottedin inFigure Figure5b. 5b.By Bycomparison comparison with measured before andafter aftercoating coatingthe theGO GO thin thin film, film, as as plotted with thethe uncoated case, in the theresonant resonantfrequency, frequency,which which might uncoated case,it itisisfound foundthat thatthere there isis aa slight slight increase increase in might be be caused by an increase in the mechanical stiffness of the pMUT after depositing GO film. caused by an increase in the mechanical stiffness of the pMUT after depositing GO film.

3.2. Humidity-Sensing Properties 3.2. Humidity-Sensing Properties The pMUT humidity sensors were tested in the well-sealed plastic chamber with the relative The pMUT humidity sensors were tested in the well-sealed plastic chamber with the relative humidity 10%toto90% 90% RH. frequency shift to different humidity levels is humidityincreasing increasing from from 10% RH. TheThe frequency shift to different humidity levels is shown shown in Figure 6a,b. the When thesensors pMUTwere sensors weretoexposed to the low humidity condition, in Figure 6a,b. When pMUT exposed the low humidity condition, there is an there is an approximately linear relationship between the frequency shift and the humidity levels. approximately linear relationship between the frequency shift and the humidity levels. However, However, when the humidity is continuously increased to a high RH level, a nonlinear decrease when the humidity is continuously increased to a high RH level, a nonlinear decrease in in thethe resonant frequency good accordance accordancewith withthe theprevious previous reports [19,26]. resonant frequencycan canbe beobserved, observed, which which is is in in good reports [19,26]. The whole absorption in the the GO GOfilm filmcan canbebedivided dividedinto into two steps: The whole absorptionprocess processof of water water molecules molecules in two steps: superficial adsorption at the low humidity level and volumetric adsorption at the high humidity level. superficial adsorption at the low humidity level and volumetric adsorption at the high humidity level. the frequency shiftrespect Δf withtorespect to thehumidity relative levels humidity canand be described fitted and by Thus, theThus, frequency shift ∆f with the relative can levels be fitted the following expression: thedescribed followingbyexpression: c· RH (3) |∆ f |Δ= f =a a· ⋅RH RH + + bb⋅ ·e ce⋅ RH + d+ d (3) where a, b, c, and d are the fitting coefficients and they are estimated correspondingly as 0.0979, 2.551, where a, b, c, and d are the fitting coefficients and they are estimated correspondingly as 0.0979, 0.03379, and −4.5518. 2.551, 0.03379, and −4.5518. The humidity sensitivity aredefined definedasas The humidity sensitivitySSand andrelative relativehumidity humidity sensitivity sensitivity SSRRare SS==

Δff | |∆ ∆RH ΔRH

SS = Δ |∆f f | SRS= = R = RH f 0f 0 ff00⋅·Δ∆RH

(4) (4) (5) (5)

where f 0 fis the the pMUT pMUThumidity humiditysensor sensorwith with the GO thin film where 0 is thefundamental fundamentalresonant resonant frequency frequency of the the GO thin film at at 10% RH. Therefore, ofthe thepMUT pMUThumidity humiditysensor sensor 10% RH. Therefore,the thesensitivity sensitivity and and relative relative sensitivity sensitivity of in in thethe